1. Field of the Invention
The present invention relates to ferroelectric thin-film devices having a highly oriented ferroelectric thin-film disposed on a substrate, and particularly relates to an oriented ferroelectric thin-film device applicable to capacitors for DRAM and ferroelectric RAM, pyroelectric devices, microactuators, thin film capacitors, compact piezoelectric devices and other suitable devices. The present invention also relates to a method for manufacturing the oriented ferroelectric thin-film device.
2. Description of the Related Art
Studies have recently been conducted on oriented ferroelectric thin-film devices having a ferroelectric thin-film disposed on a single crystal substrate, wherein the ferroelectric thin-film comprises a Pb-containing perovskite compound such as PZT and PLZT having large residual polarization. In the oriented ferroelectric thin-film devices, spontaneous polarization can be arranged in one direction, thereby obtaining higher polarization values and switching characteristics. Therefore, it is expected that the application of the oriented ferroelectric thin-film devices to a high-density recording medium be significantly increased.
Thus, research and development has been actively conducted on a method for forming an oriented thin-film comprising a Pb-containing perovskite compound such as PbTiO3, (Pb, La)TiO3 (PLT), PZT, PLZT, and Pb(Mg, Nb)O3 (PMN) on a single crystal substrate. Such a method is currently being used.
However, it is not necessarily easy to obtain the above oriented ferroelectric thin-film device in which a ferroelectric oxide thin-film having excellent crystallinity and uniaxial orientation is disposed on a single crystal substrate. The reasons are described below.
(1) Since a Pb-containing perovskite ferroelectric is formed only at a temperature higher than its phase transition temperature, a 90° domain is formed so as to relieve the stress caused by the phase transition during a cooling step after forming the thin-film. Applying compression stress to thin-films is generally effective in order to suppress the formation of the 90° domain. Thus, a substrate having a large temperature coefficient of expansion is usually employed to apply compression stress to thin-films by using the thermal contractive force of the substrate during a cooling step after forming the thin-films. However, when the single crystal substrate comprises Si or Al2O3 having a small thermal contractive force, this method is not effective because compression stress cannot sufficiently be applied to the thin-film and it is thus difficult to prevent the formation of the 90° domain.
(2) A method for forming an oriented PZT layer on an Si substrate having Pt—Ti electrodes disposed thereon is proposed in Jpn. J. Appl. Phys. Vol. 37 (1998), L5128. The method in which a template layer comprising PbTiO3 is formed on a Pt—Ti—Si substrate by a laser ablation process, that is, a pulsed laser deposition process, to grow an oriented PZT layer by a sol-gel method. However, according to this method, there is a problem in that cost of providing vacuum equipment such as a deposition chamber is high and production cost is also high due to an increase in the number of production steps because ultra high vacuum is required to form the PbTiO3 layer. Furthermore, the production cost is increased due to the use of expensive single crystal substrates.